The invention relates to well treating fluids for use in the drilling and servicing of wells in subterranean formations. More particularly, it relates to saturated brine fluids having suspended therein one or more water soluble salts which are insoluble in the saturated brine. Such fluids are useful in various well drilling, workover, and completion operations.
Additionally, the invention relates to blends of xanthomonas gum and hydroxypropyl ether of starch which has been crosslinked with epichlorohydrin. These blends exhibit high suspending capacity and enchanced fluid loss control in saturated saline solutions.
After a well has been drilled into the earth's surface, one or more subterranean producing formations may be encountered. It is then desirable to complete the well so as to obtain the maximum hydrocarbon production from such subterranean producing formations, and to complete such well in a manner so that the pressure in the subterranean producing formations is controlled during completion operations, while bridging or sealing off the producing formation to inhibit damage thereto and to minimize fluid loss into the formation which might inhibit or in some cases substantially reduce the production of hydrocarbons from the subterranean producing formation.
Also, it is desirable or necessary after a period of time to perform workover or other service operations on a previously drilled well, and during such workover or service operations, it is desirable to control the pressure in the subterranean producing formation or formations while at the same time bridging or sealing off the formations in a manner so as to minimize particle invasion and fluid loss into the producing formation or formations and thereby reduce if not substantially eliminate damage to the formations.
In addition to the necessary required density characteristics of a workover and completion fluid, it is desirable to seal off or temporarily plug the face of the subterranean producing formation or formations in the well bore so that during the completion and workover operation fluid and solids in the fluid are not lost to the producing formation which might cause damage thereto.
Heretofore various types of workover and completion fluids with components therein of a particle size to bridge and seal off the producing formations have been employed. Workover and completion fluids should have fluid loss (filter loss) control to prevent substantial fluid invasion of the formation. This is achieved by a combination of a fluid loss control agent and a bridging agent.
Various types of soluble or degradable bridging materials are available commercially, and the choice between them depends on reservoir conditions and type of operation. Sized particles of oil-soluble resins or waxes have been used as bridging agents for oil reservoirs. Such particles must be removed by dissolving them in oil when used in dry gas reservoirs or water injection wells. Calcium carbonates were the first degradable bridging particles to be used in workover and completion fluids. On completion of the job, they are removed with acid if necessary. Acidization is an extra operation and additional expense, except in carbonate reservoirs that must be acidized. Furthermore, the acid may dissolve iron on the way down to the acidizing zone and introduce iron compounds into the formation. Then, when the acid is spent, the pH rises, iron hydroxide is precipitated, and formation damage results. All of the carbonate particles may not be contacted by the acid. To avoid this problem, alternate slugs of acid and diverting agent are necessary.
Workover and completion fluids can be formulated utilizing sized particles of a water soluble salt as a bridging agent suspended in a saturated brinc solution in which the salt is substantially insoluable. See U.S. Pat. Nos. 4,175,042; 4,186,803; and 4,369,843.
These treating fluids comprise a saturated aqueous saline solution with at least one water soluble salt which is substantially insoluble in the saturated saline solution, the water soluble salt having a particle size range of about 5 microns to about 800 microns with greater than 5 percent of the particles being coarser than 44 microns to control the pressure in the formation while bridging and sealing it, and also minimizing particle invasion to the formation. A minor amount of fluid loss additive to inhibit loss of fluid into the subterranean producing formation, and a suspension additive to prevent settling of the water soluble salt particles is included in the treating fluid. The water soluble salt which is substantially insoluble in the saline solution is added in sufficient quantity to control the pressure in the producing formation by increasing the density of the treating fluid.
The bridging and sealing salt particles used in the well completion and workover method can be dissolved by the flow of produced field brine or by the injection of water or an unsaturated saline solution. This eliminates the undesirable use of oil or acid solution to remove the bridge from the subterranean hydrocarbon producing formation. The saturated saline solution is formed by dissolving a salt or mixture of salts in water and normally the minimum density of the saturated saline solution is approximately at least 1.2 grams per cubic centimeter.
It is also disclosed in Gruesbeck et al. U.S. Pat. No. 4,046,197 to suspend finely divided particles of a water soluble salt in a saturated aqueous saline solution having a density of at least about 1.2 g/cc at 10.degree. C. in an amount sufficient to substantially increase the overall density of the fluid. Preferably the average size of the suspended salt particles is less than about 10 microns. Mondshine U.S. Pat. No. 4,621,692 discloses a method of protecting the perforations during workover and completion operations utilizing a saturated saline solution having sized water soluble salt bridging particles suspended therein.